As well known, vehicles such as an electric motorcar and a hybrid car include motors which are source of power and high voltage and large capacity battery which supplies electric power to the motors. The high voltage battery contains plural battery cells which are lithium ion batteries, hydrogen nickel batteries, or the like and which are series-connected.
The high voltage battery is insulated from a chassis ground for safety. Thus, it is very important to monitor the insulating state of the high voltage battery and the chassis ground. For example, the patent document 1 discloses the technique which measures the insulating state (specifically insulation resistance) of the high voltage battery and the chassis ground by using the insulated measuring circuit of a flying capacitor method.